Colligative Properties of Solutions

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Colligative Properties of Solutions

• Chemistry
• Mrs. Coyle

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How do you get from this
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to this?
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Add an ionic compound!
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Colligative Properties

• Properties that depend only on the number of
  solute particles and not on their identity.

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Some Colligative Properties are

• Vapor pressure lowering
• Boiling point elevation
• Freezing Point depression

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Vapor Pressure
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Vapor Pressure Lowering

• The particles of solute are surrounded by and
  attracted to particles of solvent.
• Now the solvent particles have less kinetic
  energy and tend less to escape into the space
  above the liquid.
• So the vapor pressure is less.

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Ionic vs Molecular Solutes

• Ionic solutes produce two or more ion particles
  in solution.
• They affect the colligative properties
  proportionately more than molecular solutes (that
  do not ionize).
• The effect is proportional to the number of
  particles of the solute in the solution.

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How many particles do each of the following give
upon solvation?

• NaCl
• CaCl2
• Glucose

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Freezing Point Depression
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Example

• Salt is added to melt ice by reducing the
  freezing point of water.

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Boiling Point Elevation
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Example

• Addition of ethylene glycol C2H6O2 (antifreeze)
  to car radiators.

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Freezing Point Depression and Boiling Point
Elevation

• Boiling Point Elevation
• ?Tb mkb (for water kb0.51 oC/m)
• Freezing Point Depression
• ?Tfmkf (for water kf1.86 oC/m)
• Note m is the molality of the particles, so if
  the solute is ionic, multiply by the of
  particles it dissociates to.

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Which is more effective for lowering the freezing
point of water?

• NaCl or CaCl2

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Example 1

• Find the new freezing point of 3m NaCl in water.

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Example 2

• Find the new boiling point of 3m NaCl in water.